Vision par ordinateur

Transcription

1 Vision par ordinateur Géométrie épipolaire Frédéric Devernay Avec des transparents de Marc Pollefeys

2 Epipolar geometry π Underlying structure in set of matches for rigid scenes C1 m1 l1 M L2 L1 l T 1 l 2 e1 m 2 T F m 1 = 0 e2 Fundamental matrix 3x3 rank 2 matrix) m2 C2 l2 Canonical representation: P = [I 0] " P = [[ " e ] # F + " e v T " e ] 1. Computable from corresponding points 2. Simplifies matching 3. Allows to detect wrong matches 4. Related to calibration

3 Epipolar geometry x1 C1 l1 ΠP l 1 = e 1 " x 1 L2 " = P T 1 l 1 = P T 2 l 2 M L1 l T 1 l 2 e1 x T 2 Px + T 2 2 P 1 F [ex 11 ] " = x 1 0= 0 e2 x2 l2 Fundamental matrix 3x3 rank 2 matrix) C2 l 2 T x 2 = 0 l 2 = P 2 +T P 1 T l 1

4 The projective reconstruction theorem If a set of point correspondences in two views determine the fundamental matrix uniquely, then the scene and cameras may be reconstructed from these correspondences alone, and any two such reconstructions from these correspondences are projectively equivalent allows reconstruction from pair of uncalibrated images!

5 Properties of the fundamental matrix

6 Computation of F Linear 8-point) Minimal 7-point) Robust RANSAC) Non-linear refinement MLE, ) Practical approach

7 Epipolar geometry: basic equation " x T Fx = 0 x " xf 11 + x " yf 12 + x " f 13 + y " xf 21 + y " yf 22 + y " f 23 + xf 31 + yf 32 + f 33 = 0 separate known from unknown [ x " x, x " y, x ", y " x, y " y, y ", x, y,1] f 11, f 12, f 13, f 21, f 22, f 23, f 31, f 32, f 33 data) [ ] T = 0 unknowns) linear) y 1 x 1 y 1 1& f = 0 y n x n y n 1' # x 1 " x 1 x 1 " y 1 x 1 " y 1 " x 1 y 1 " y 1 " M M M M M M M M M $ x " n x n x " n y n x " n y " n x n y " n y n " Af = 0

8 the NOT normalized 8-point algorithm # x 1 x 1 " y 1 x 1 " x 1 " x 1 y 1 " y 1 y 1 " y 1 " x 1 y 1 1& x 2 x " 2 y 2 x " 2 x " 2 x 2 y " 2 y 2 y " 2 y " 2 x 2 y 2 1 M M M M M M M M M $ x n x " n y n x " n x " n x n y " n y n y " n y " n x n y n 1' ~10000 ~10000 ~100 ~10000 ~10000 ~100 ~100 ~100 1 Orders of magnitude difference! between column of data matrix $ least-squares yields poor results # f 11 f 12 f 13 f 21 f 22 f 23 f 31 f 32 f 33 & = 0 '

9 the normalized 8-point algorithm Transform image to ~[-1,1]x[-1,1] 0,500) 700,500) # $ & 0 " "1 1 ' -1,1) 0,0) 1,1) 0,0) 700,0) -1,-1) 1,-1) normalized least squares yields good results Hartley, PAMI 97)

10 the singularity constraint e " T F = 0 Fe = 0 detf = 0 rank F = 2 SVD from linearly computed F matrix rank 3) #" 1 F = U " 2 $ " 3 Compute closest rank-2 approximation & V T = U " V T + U " V T + U " V T ' min F - F " F $ # 1 & F " = U & # 2 & ' ) ) V T = U # V T + U # V T )

11 F vs. F'

12 the minimum case 7 point correspondences # x 1 " x 1 x 1 " y 1 x 1 " y 1 " x 1 y 1 " y 1 y 1 " x 1 y 1 1& M M M M M M M M M f = 0 $ x " 7 x 7 x " 7 y 7 x " 7 y " 7 x 7 y " 7 y 7 y " 7 x 7 y 7 1' T A = U 7x7 diag " 1,...," 7,0,0)V 9x9 " A[V 8 V 9 ] = 0 9x2 e.g.v T V 8 = [ ] T ) x i T F 1 + "F 2 )x i = 0,#i =1...7 one parameter family of solutions but F 1 +λf 2 not automatically rank 2

13 the minimum case impose rank 2 σ 3 obtain 1 or 3 solutions) F 7pts F 1 F 2 F detf 1 + "F 2 ) = a 3 " 3 + a 2 " 2 + a 1 " + a 0 = 0 detf 1 + "F 2 ) = det F 2 detf 2-1 F 1 + "I) = 0 cubic equation) det AB) = det A).det B) ) F 2-1 F 1 Compute possible λ as eigenvalues of only real solutions are potential solutions) Minimal solution for calibrated cameras: 5-point

14 Robust estimation What if set of matches contains gross outliers? to keep things simple let s consider line fitting first)

15 RANSAC RANdom Sampling Consensus) Objective Robust fit of model to data set S which contains outliers Algorithm i) ii) Randomly select a sample of s data points from S and instantiate the model from this subset. Determine the set of data points S i which are within a distance threshold t of the model. The set S i is the consensus set of samples and defines the inliers of S. iii) If the subset of S i is greater than some threshold T, reestimate the model using all the points in S i and terminate iv) If the size of S i is less than T, select a new subset and repeat the above. v) After N trials the largest consensus set S i is selected, and the model is re-estimated using all the points in the subset S i

16 Distance threshold Choose t so probability for inlier is α e.g. 0.95) Often empirically 2 d! Zero-mean Gaussian noise σ then follows 2! m distribution with m=codimension of model dimension+codimension=dimension space) Codimension Model line,f H,P T t σ σ σ 2

17 How many samples? Choose N so that, with probability p, at least one random sample is free from outliers. e.g. p=0.99 1" 1" e) s ) N =1" p N = log 1" p) /log 1" 1" e) s ) s proportion of outliers e Note: Assumes that inliers allow to identify other inliers

18 Acceptable consensus set? Typically, terminate when inlier ratio reaches expected ratio of inliers T = 1" e)n

19 Adaptively determining the number of samples e is often unknown a priori, so pick worst case, i.e. 0, and adapt if more inliers are found, e.g. 80 would yield e=0.2 N=, sample_count =0 While N >sample_count repeat Choose a sample and count the number of inliers Set e=1-number of inliers)/total number of points) Recompute N from e Increment the sample_count by 1 Terminate N = log 1" p) /log 1" 1" e ) s ))

20 Other robust algorithms RANSAC maximizes number of inliers LMedS minimizes median error Not recommended: case deletion, iterative least-squares, etc.

21 Non-linear refinment

22 Geometric distance Gold standard Symmetric epipolar distance

23 Gold standard Maximum Likelihood Estimation Parameterize: P = [I 0], P " = [M t],x i = least-squares for Gaussian noise) 2 " ^ 2 " " T d$ x i,x^ ) i' + d $ x # & i, x i' subject to x^ F x^ # & i Initialize: normalized 8-point, P,P ) from F, reconstruct X i x^ i = PX i,x^ i = P " X i Minimize cost using Levenberg-Marquardt preferably sparse LM, e.g. see H&Z) = 0 overparametrized)

24 Gold standard Alternative, minimal parametrization with a=1) note x,y,1) and x,y,1) are epipoles) problems: a=0 pick largest of a,b,c,d to fix to 1 epipole at infinity pick largest of x,y,w and of x,y,w 4x3x3=36 parametrizations! reparametrize at every iteration, to be sure

25 Symmetric epipolar error # i ) 2 + d x i,f T " d x " i,fx i # = ) x " T Fx $ x ) 2 i x " T 2 F) + " 1 1 x T F) Fx ) 1 + Fx) 2 & '

26 Some experiments:

27 Some experiments:

28 Some experiments:

29 Some experiments: Residual error: # i ) 2 + d x i,f T " d x " i,fx i for all points!) x ) 2 i

30 Recommendations: 1. Do not use unnormalized algorithms 2. Quick and easy to implement: 8-point normalized 3. Better: enforce rank-2 constraint during minimization 4. Best: Maximum Likelihood Estimation minimal parameterization, sparse implementation)

31 Automatic computation of F Step 1. Extract features Step 2. Compute a set of potential matches Step 3. do Step 3.1 select minimal sample i.e. 7 matches) Step 3.2 compute solutions) for F Step 3.3 determine inliers until Γ#inliers,#samples)<95 verify hypothesis) } generate hypothesis) Step 4. Compute F based on all inliers Step 5. Look for additional matches Step 6. Refine F based on all correct matches " =1# 1# # inliers ) 7 ) # samples # matches #inliers #samples

32 Two-view geometry geometric relations between two views is fully described by recovered 3x3 matrix F